X-ray source assemblies are known in the art, and have been widely recognized as being extremely valuable tools for use in a variety of applications, ranging from the medical field to, more recently, the detection of explosives. With respect to the medical field, for example, such assemblies are commonly used in areas such as diagnostic and therapeutic radiology. With respect to explosives detection, such assemblies have more recently been adapted for use within detection equipment produced by the Assignee of this invention for use in detecting explosives in luggage moving on airline luggage conveyors and the like. Examples of such equipment are defined in the foregoing co-pending patent applications.
Generally speaking, the operation of x-ray source assemblies such as those defined herein is similar. In general, x-rays, or x-ray radiation, are produced when electrons are produced, accelerated to high speeds, and then stopped abruptly. Typically, this procedure occurs within an x-ray tube which includes an evacuated envelope which is usually comprised of glass or a combination of metal and glass. A cathode structure is typically located within the envelope for producing the electrons when the tube is activated. An anode structure is located at a short spaced location (gap) from the cathode structure and designed for receiving electrons emitted by the cathode. A voltage potential is applied between the cathode and the anode which causes the electrons emitted from the cathode structure (also called a filament) to form a thin stream of beams which are accelerated to a very high velocity towards a surface (also called a target) on the anode structure. This anode target surface is comprised of a refractory metal having a high atomic number, so that when the electrons strike it, at least a portion of the resulting kinetic energy is converted to electromagnetic waves of very high frequency, these being the x-rays. The resulting x-rays emanate from the target surface, and are then collimated for penetration into an object, such as the aforementioned luggage moving along a conveyor. As mentioned, when used within explosive detection equipment, these x-rays pass through the luggage, are detected and then analyzed so as to determine the nature of the object or objects within the luggage.
Generally speaking, a relatively minor part of the input energy provided the x-ray assembly results in the production of x-rays. A majority of the kinetic energy resulting from the electron collisions at the target surface is converted into heat, which can reach extremely high temperatures. The heat is absorbed by the anode and is conducted not only to other portions of the anode assembly, but to the other x-ray tube components within the evacuated envelope and, also, to the other parts of the x-ray assembly holding the x-ray tube therein. Over time, this heat can damage the anode, the anode assembly, other tube components and/or other internal parts of the overall assembly, and can reduce the operating life of the x-ray tube and/or the performance and operating efficiency of the tube.
Several approaches have been used to help alleviate problems arising from the presence of these high operating temperatures. For example, in some x-ray devices, the x-ray target, or focal track, is positioned on an annular portion of a rotational anode disk. The anode is in turn mounted on a supporting shaft and rotor assembly that can then be rotated by a motor. During operation, the disk is rotated at high speeds, causing the focal track to continuously rotate into and out of the path of the electron beam such that the electron beam is in contact with any given point along the focal track for only short periods of time. This allows the remaining portion of the track to cool during the time that it takes to rotate back into the path of the electron beam, thereby reducing the amount of heat absorbed by the anode. Although anode rotation reduces the amount of heat present at the focal spot on the focal track, a large amount of heat is still transferred to the anode, the anode drive assembly, and other components within the assembly housing.
Another approach has been to place the housing that forms the evacuated envelope within a second outer metal (e.g., lead) housing which acts as a radiation shield to prevent radiation leakage and further serves as a container for a cooling medium, one well known such medium being oil (also referred to as dielectric oil due to its lack of electrical conductive properties). It is known in the art to use such oil and circulate it by a pump over the outer surface of the inner evacuated housing. As heat is emitted from the x-ray tube components (anode, anode drive assembly (if used), etc.), it is radiated to the outer surface of the evacuated housing, and then at least partially absorbed by the coolant fluid. The heated coolant fluid is then passed to some form of heat exchange device, such as a radiating surface, to allow much of the heat to be removed. The fluid is then re-circulated by the pump back through the outer housing and the process repeated. Oil may also serve as an electrical insulator and reduce the possibility of electrical arcing between the evacuated housing and the outer housing. It is thus essential that the oil be properly circulated within the x-ray assembly.
Some known x-ray sources have eliminated the use of an outer housing and oil as a coolant/dielectric medium. For example, some solutions utilize forced air to remove heat from the evacuated housing and its components. However, these approaches have not been entirely satisfactory for a variety of reasons. For example, known x-ray generating devices that utilize forced air as a cooling medium are adapted for high voltage x-ray applications; such applications typically utilize a 150 kV operating potential, or higher, between the anode and cathode. High operating voltages result in higher operating temperatures, and to ensure sufficient heat removal with air convection, these x-ray tubes typically are equipped with fins, or channels formed on the outer surface of the evacuated envelope so as to enhance heat removal. Understandably, this need for additional structure increases manufacturing complexity, and involves additional physical space requirements for the assembly.
Another concern related to x-ray assemblies and particularly those which operate at relatively high voltages (e.g., those above 150 kV) is the potential for unacceptable arcing between the anode and cathode structures. Such arcs can of course destroy the inner components of the x-ray tube and cause other damage to the overall system. X-Ray tubes have a high internal vacuum and, as a result, the metal internal parts may contaminate the vacuum when the tube is not in use. Normally, such contamination is recombined during the tube's normal operation. If this does not successfully occur, however, the contamination may cause what is referred to as “micro-arcing” or even a major arc. In most cases, micro arcs merely cause an anomaly in the tube's detected data and will self-cure as the tube continues to operate. A major arc, however, will further contaminate the vacuum each time it occurs. Running the tube for a given time period (depending on the extent of arcing damage) will degas the tube so that it can be returned to full service. If the damage is extensive, however, the tube will require replacement. In conventional tube designs, the power supply serves to detect arcs, as evidenced by excessive current flow.
Examples of various x-ray assemblies are described in the following documents.
In U.S. Pat. No. 3,473,028, there is described a relatively small radiographic examination apparatus which includes a housing having walls of a dielectric material, an electrically conductive liner secured to the interior surface of the housing walls and inflow and outflow means communicating with the interior chamber of the housing for permitting the free circulation of an insulating, dielectric medium to and from said chamber.
In U.S. Pat. No. 4,079,217, there is described an apparatus for improving the life of metallic bellows used in a hermetically sealed device having a movable element therein such as in vacuum relays and circuit breakers. The convolutions of the metal bellows are filled by a silicone dielectric gel for an axial length including several such convolutions adjacent the movable end of the bellows, thereby allegedly damping axial mechanical vibrations and preventing excessive stress build-up in the bellows portions adjacent the moveable end.
In U.S. Pat. No. 4,127,776, there is described a conventional x-ray tube and shield assembly for use in dental radiography. The conventional assembly has the x-ray tube affixed within an open-ended cylindrical shield such that the focal point of the tube is centered with respect to a small opening provided in the shield. Positioning of this tube-shield assembly within an x-ray generator is accomplished by trial and error means. The improvement includes an interiorly threaded tubular member extending outwardly from the shield and concentrically around the small opening of the shield. The threaded tubular member is received by a filter element which is disposed within an eye-port opening of an x-ray generator. Since a close dimensional clearance is provided between the tubular member and eye-port opening, the tubular member thus automatically self-centers the x-ray tube and shield within the x-ray generator when the filter element engages the tubular member.
In U.S. Pat. No. 4,355,410, there is described an enclosed, self-contained, air-cooled industrial x-ray machine having a housing which is also a gas-to-gas heat exchanger. The cylindrical metallic housing for the x-ray tube and power transformers is machined to provide a large plurality of narrow radial grooves with intervening narrow vanes on both the inside surface and the outside surface. The outside is covered with a thin-walled cylindrical jacket to provide a plurality of longitudinal passageways. An inside tubular sleeve provides support for the x-ray tube and is adapted to fit closely inside of the inner grooves to provide a plurality of longitudinal passageways on the inner surface. The housing is closed off and sealed with end plates and the interior is filled with a selected heat-transfer and insulating gas at a selected pressure. An internal fan provides circulation of the gas over the x-ray tube and back to the inner longitudinal passageways to the fan. An outside fan circulates room air through the outer longitudinal passageways.
In U.S. Pat. No. 4,841,557, there is described an x-radiator having heat-producing components therein which are enclosed in a housing filled with an insulating fluid surrounding the components. A circulating pump is in fluid communication with the interior of the housing and has respective ports through which the fluid is circulated through the pump between the ports for aiding in dissipating heat from the components. The pump may be integrated within the housing, or attached to an exterior of the housing, with only its ports being in communication with the housing interior. The pump may be a squirrel-cage induction motor with a fluid conveying element, such as a ship's propeller, forming a unit with the rotor and being arranged in a protective housing together with the stator.
In U.S. Pat. No. 4,884,292, there is described an x-ray tube generally comprising an evacuated tube envelope fabricated primarily of metal and having first and second end walls and a cylindrical sidewall, enclosing a rotating anode with its target surface facing the second end wall. A heat transfer sleeve extends from the first end wall past the anode to receive heat from the anode and transfer it to the end wall for dispersal. A heat transfer cross plate at the end of the sleeve further encloses the anode to receive and transfer heat. The tube envelope is mounted within a cylindrical tube housing with the first end wall in contact with a finned mounting plate for dissipating heat. The surface of the tube envelope remains sufficiently cool to apply a layer of lead, whereby the tube is compact. The anode and cathode electrical feeds are through the second end wall. The electrical feeds have angled terminations with lead thereon.
In U.S. Pat. No. 5,086,449, there is described an x-ray tube in which oil is circulated through a heat exchanger to reduce its temperature. More specifically, at least one hot coolant fluid receiving aperture is defined adjacent an end of a suction tube in an upper most portion of a horn portion surrounding a cathode termination assembly. Bubbles of gas in the fluid which could be ionized by electrical fields inside the x-ray tube housing causing x-ray tube current irregularities and corresponding x-ray tube output irregularities are drawn into the suction tube aperture. A de-bubbler removes bubbles from the cooled coolant fluid before it is returned into an anode horn portion of the x-ray tube. Alternatively, the bubbles may be re-absorbed, dissolved, or homogenized by the action of the heat exchanger and pump. The coolant fluid passes through a central portion of the x-ray tube absorbing heat and back to the cathode horn portion.
In U.S. Pat. No. 5,357,555, there is described a method for operating an x-ray installation having an x-ray radiator which comprises an x-ray tube located in a housing filled with an electrically insulating liquid. The electrically insulating liquid is thereby degasified at intervals in order to prevent gases arising as a consequence of the decomposition of the electrically insulating liquid caused by the generated x-ray radiation from deteriorating the high-voltage strength of the x-ray radiator.
In U.S. Pat. No. 5,802,140, there is described an x-ray generating apparatus which is provided with a unitary vacuum enclosure having a rotating anode target and a cathode assembly for generating x-rays transmitted through an x-ray window. The cathode assembly is placed within the vacuum enclosure through an opening in the top wall thereof, and comprises a disk which completely covers this opening. The unitary vacuum enclosure and the disk form a radiation shield. For increasing a thermal capacity of the unitary vacuum enclosure and installing the x-ray generating apparatus into a gantry, it further comprises a mounting block which may be coupled to or encompass the unitary vacuum enclosure. The x-ray window is placed within the mounting block. A window adaptor may be utilized for the X-ray window installation.
In U.S. Pat. No. 6,254,272, there are described methods and apparatus for allegedly extending the life of an x-ray tube which contains an “insert” for generating x-rays. The insert is housed in a housing wherein an insulating fluid circulates around the insert in the housing to provide thermal and electrical insulation. This patent includes methods and apparatus for removing water from insulating oil. One embodiment includes a processor containing a coalescing element for removing water as a vapor from the oil. Other embodiments include methods and devices for drying the interior of the housing. Another embodiment includes a kit containing a processor having a coalescing element for removing water from the insulating oil.
In U.S. Pat. No. 6,487,273, there is described a radiographic apparatus that utilizes a single integral housing for providing an evacuated envelope for an anode and cathode assembly. The integral housing allegedly provides sufficient radiation blocking and heat transfer characteristics such that an additional external housing is not required. The integral housing is air cooled, and thus does not utilize any coolant. In addition, the integral housing is insulated with a potting material, which electrically insulates the integral housing and its components, and also limits the amount of noise emitted from the housing during operation. In an alternative embodiment, enhanced thermal and electrically insulating properties are allegedly achieved through the use of a potting material disposed in selected areas of the tube interior. The potting material cooperates with optimized airflow through the tube assembly to effectively and continuously remove heat there-from.
In U.S. Pat. No. 6,494,618, there are described devices for improved radiation attenuation in devices which generate x-ray radiation. A high voltage receptacle is disclosed, the receptacle being adapted to accommodate a high voltage connector to supply power to an x-ray tube and being formed of a mixture of a dielectric material and an x-ray attenuating material, such as an x-ray attenuating metal compound. X-ray radiation impinging upon the high voltage receptacle that would otherwise pass through the unshielded receptacle is absorbed or scattered by the x-ray attenuating material without the need for additional x-ray shielding. Also disclosed is an x-ray housing assembly including an x-ray housing adapted to contain an x-ray tube, and a high voltage receptacle, wherein the high-voltage receptacle and optionally a portion of the x-ray housing is formed of a mixture of a dielectric material and an x-ray attenuating material.
In Printed Patent Application Number US2002/0020547 A1, there is described a high-voltage generator which is proportioned so as to realize a particularly low weight in combination with a high output power and is suitable notably for use in rotating x-ray systems such as computed tomography apparatus. To this end, the high-voltage generator is provided with a hybrid insulation which is formed as far as possible by high resistance foam and an insulating liquid. The foam is shaped and arranged in such a manner that there are formed channels through which the insulating liquid can flow in areas requiring a discharge of heat or an electric strength stronger than can be ensured by the high-resistance foam alone.
In Printed Patent Application Number US2002/0196905 A1, the useful life of x-ray tubes is allegedly extended by filtering metal particles and other decomposition products out of the coolant fluid by filter means permanently included in the closed loop cooling fluid circuit which also includes pump means and heat exchange means.
In Printed Patent Application Number US2005/0232395 A1, there is described a dielectric connector for use in high voltage devices, including x-ray tubes. The connector comprises a dielectric material and is pre-formed before attachment to the x-ray tube. Pre-formation of the connector creates a first cavity portion therein that conforms in shape to a corresponding segment of the tube surface. A second cavity portion is also defined for receiving a high voltage receptacle. Upon attachment to the tube, the first cavity portion receives the corresponding tube segment. The high voltage receptacle is received into the second cavity portion and electrically connects with a receptacle defined on the tube surface. The receptacle enables a high voltage signal passing through the electrode to connect with either the anode or cathode disposed within the tube. Pre-formation of the connector enables connector testing and repair to occur before it is attached to the tube, saving resources, time, and cost.
As understood from the following, the present invention represents an improvement to known x-ray source assemblies in which an x-ray tube is used and located within a housing. The present invention provides for enhanced cooling of the x-ray tube and other internal parts of the assembly while also assuring an effective means of electrically coupling the x-ray tube to the necessary electrical connections through which the x-ray tube is electrically powered. Still further, the invention provides for means for detecting for undesirable arcing between the anode and cathode structures of the x-ray tube to thereby inform the assembly operator of potential harm or destruction of the assembly. Additional features of the invention will be readily discernible from the following detailed description.
It is believed that an x-ray source assembly having such desirable features and others discernible from the following teachings will represent a significant advancement in the art.